Copper nanoparticles coated by reducing agent

Abstract

Provided are copper nanoparticles that: (I) have excellent dispersibility in an organic solvent and thus makes a paste containing the copper nanoparticles from which a highly smooth coating film can be formed; and (II) have excellent sinterability at low temperatures. At least a portion of the surface layer of each of the copper nanoparticles coated by a reducing agent is a film that contains copper oxide and optionally contains copper carbonate. The surfaces of the copper nanoparticles are coated with a reducing agent, where the mass carbon concentration is 0.10% to 2.00%.

Description

Reducing agent-coated copper nanoparticles

[0001] The present invention relates to reducing agent coated copper nanoparticles.

[0002] It is known that inks or pastes containing copper nanoparticles as their primary component are used as bonding materials for various electronic components and as materials for forming high-density wiring on printed wiring boards and the like. It is known that by coating the surfaces of the copper nanoparticles with a polymer film such as a dispersant to improve their dispersibility in organic solvents, it is possible to form highly smooth coating films from the inks or pastes containing copper nanoparticles as their primary component. It is also known that adding a reducing agent to the inks or pastes containing copper nanoparticles as their primary component makes it possible to sinter the copper nanoparticles at low temperatures.

[0003] Patent Document 1 (claim 5) describes a method for producing copper nanoparticle ink, which includes "(1) a step of bonding a silane coupling agent having a polymerizable functional group to the surface oxide layer of copper nanoparticles or the surface of copper oxide nanoparticles by a coupling reaction, (2) a step of reacting the bonded polymerizable functional group with a monomer to form a graft polymer chain to obtain composite particles, and (3) a step of dispersing the obtained composite particles in a solvent." This method improves the dispersibility of copper nanoparticles by imparting dispersion properties to the nanoparticles themselves in a solvent.

[0004] Patent Document 2 (claim 1) describes a method for producing a copper microparticle dispersion, which comprises adding a reducing agent, an aliphatic monocarboxylic acid as a protective agent, and a copper compound to a solvent, dispersing the copper compound in a solid state in the solvent, reducing the copper compound dispersed in the solvent to form copper microparticles whose surfaces are coated with the aliphatic monocarboxylic acid, and dispersing the copper microparticles in the solvent. This method involves coating the surfaces of copper nanoparticles with an aliphatic monocarboxylic acid, which is a fluxing agent, and sintering the copper nanoparticles at a low temperature.

[0005] JP 2017-165796 A JP 2013-47365 A

[0006] However, in the examples of Patent Document 1, the proportion of polymer chains is as high as 2.8 to 7.0 mass %, and when fired at a low temperature, the polymer chains of the electrode film are not sufficiently removed, inhibiting sintering, which raises concerns about poor adhesion and poor conductivity of the electrode film.

[0007] Furthermore, in the examples of Patent Document 2, the amount of aliphatic monocarboxylic acid added is as high as 17% by mass relative to the total mass of the copper oxide particles, which raises concerns that outgassing during sintering may cause defects such as voids and cracks when the ink or paste is fired. Furthermore, because no dispersant is added, the ink does not disperse well in organic solvents, making it difficult to form a smooth electrode film.

[0008] In view of the above problems, the present invention provides copper nanoparticles that (I) have excellent dispersibility in organic solvents, allowing a highly smooth coating film to be formed from a paste containing copper nanoparticles, and (II) have excellent sinterability at low temperatures. An object of the present invention is to provide copper nanoparticles.

[0009] In order to solve the above problems, the present inventors have conducted extensive research and have found that the above problems (I) and (II) can be solved by coating the surface of copper nanoparticles with a reducing agent that also functions as a dispersant and by setting the mass carbon concentration within a predetermined range.

[0010] The gist of the present invention, which was completed based on the above findings, is as follows: [1] Reducing agent-coated copper nanoparticles, at least a portion of whose surface layer is a coating containing copper oxide and optionally copper carbonate, the surface of which is coated with a reducing agent, and the mass carbon concentration is 0.10% or more and 2.00% or less.

[0011] [2] The reducing agent-coated copper nanoparticles according to [1] above, having a mass nitrogen concentration of 0.50% or less.

[0012] [3] The reducing agent-coated copper nanoparticles according to [1] or [2] above, having a mass oxygen concentration of 0.50% or more and 2.50% or less.

[0013] [4] The reducing agent comprises one or more selected from the group consisting of alkanolamines, alkylamines, hydrosilanes, and amine-based silane coupling agents. [1] The reducing agent-coated copper nanoparticles according to any one of [3] to [4].

[0014] [5] The reducing agent-coated copper nanoparticles according to any one of [1] to [4] above, wherein the average particle diameter of the copper nanoparticles is 50 nm or more and 500 nm or less.

[0015] The reducing agent-coated copper nanoparticles of the present invention (I) have excellent dispersibility in organic solvents, allowing a highly smooth coating film to be formed from a paste containing the copper nanoparticles, and (II) have excellent sinterability at low temperatures.

[0016] [Reducing Agent Coated Copper Nanoparticles] The reducing agent coated copper nanoparticles according to one embodiment of the present invention are copper nanoparticles whose surface is a coating at least partly containing copper oxide and optionally containing copper carbonate, and whose surface is coated with a reducing agent.

[0017] (Copper nanoparticles) The copper nanoparticles have a surface layer having at least a coating containing copper oxide. The copper oxide may be copper (I) oxide (chemical formula: Cu ２ Examples of copper oxide include copper(II) oxide (chemical formula: CuO), copper(II) oxide (also called cuprous oxide), copper(II) oxide (chemical formula: CuO), and mixtures thereof. When the copper oxide is a mixture of copper(I) oxide and copper(II) oxide, the mass ratio between them is not particularly limited. Generally, when the surface of copper nanoparticles is oxidized, a coating containing copper oxide is inevitably formed on at least a portion of the surface layer. The coating containing copper oxide preferably further contains copper carbonate. When the coating contains copper carbonate, the sintering temperature of the copper nanoparticles can be kept low. The thickness of the coating is not particularly limited, but the thickness of the coating for copper nanoparticles of a general size is about several nanometers (2 to 5 nm).

[0018] The shape (form) of the copper nanoparticles is not particularly limited. Examples of the shape of the copper particles include spherical (sphere), ellipsoid (ellipsoid), and plate-like shapes. Among these, spherical and ellipsoid shapes are preferred, and spherical shapes are more preferred.

[0019] If the size of the copper nanoparticles is too small, the surface energy increases as the specific surface area of ​​the copper nanoparticles increases, making it difficult to disperse the copper nanoparticles and uniformly coating the surface of the copper nanoparticles with a reducing agent. For this reason, the average particle diameter of the copper nanoparticles is preferably 50 nm or more, more preferably 100 nm or more. On the other hand, if the size of the copper nanoparticles is too large, the weight per particle increases, so the steric hindrance caused by the reducing agent coated on the surface of the copper nanoparticles does not function sufficiently, and the dispersibility of the reducing agent-coated copper nanoparticles when made into a paste tends to decrease. For this reason, the average particle diameter of the copper nanoparticles is preferably 500 nm or less, more preferably 200 nm or less.

[0020] The average particle size of copper nanoparticles can be calculated based on the following formula: The specific surface area of ​​the copper nanoparticles in the formula can be measured using a nitrogen gas BET adsorption device (for example, "MACSORB HM-1201" manufactured by Mountech Co., Ltd.).

[0021] Examples of such copper nanoparticles include ultrafine metal powder obtained by the manufacturing method described in Japanese Patent No. 4304221 and copper microparticles obtained by the manufacturing method described in Japanese Patent No. 6130616 (dry method using a reducing flame). Also, if copper nanoparticles are commercially available, they may be used. Copper nanoparticles manufactured by a dry method are not limited to the above manufacturing method. For example, copper nanoparticles can also be obtained by a method of reducing a copper compound gas with a reducing gas or a method of cooling copper vaporized by plasma.

[0022] (Reducing Agent) The reducing agent is a compound that reduces copper oxide, which is inevitably present in the coating on the surface of copper nanoparticles during sintering. During sintering, the reducing agent removes the coating containing copper oxide, causing the copper particles (pure copper) to come into contact with each other, leading to sintering and diffusion bonding. In this embodiment, the reducing agent also functions as a dispersant that improves dispersibility in organic solvents.

[0023] The reducing agent is not particularly limited as long as it interacts with the surface of the copper nanoparticles, can be adsorbed or chemically bonded to the surface of the copper nanoparticles, improves dispersibility in a solvent, and acts as a reducing agent. Examples of such reducing agents include one or more selected from the group consisting of alkanolamines, alkylamines, hydrosilanes, and amine-based silane coupling agents.

[0024] Examples of alkanolamines include monoethanolamine, diethanolamine, and triethanolamine.

[0025] Examples of alkylamines include ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, triethylamine, tripropylamine, tributylamine, tripentylamine, and trihexylamine.

[0026] Examples of hydrosilanes include dimethoxymethylsilane, diethoxymethylsilane, trimethoxysilane, and triethoxysilane.

[0027] Examples of the amine-based silane coupling agent include aminopropyltrimethoxysilane, aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, and 3-(2-aminoethylamino)propyltriethoxysilane.

[0028] (Mass Carbon Concentration) In order to obtain the effects of (I) excellent dispersibility in organic solvents, allowing a highly smooth coating film to be formed from a paste containing the copper nanoparticles, and (II) excellent sinterability at low temperatures, it is important that the reducing agent-coated copper nanoparticles of this embodiment have a mass carbon concentration of 0.10% to 2.00%. Here, "mass carbon concentration" refers to the mass ratio (mass%) of carbon to the total mass of the reducing agent-coated copper nanoparticles, and is the sum of the carbon concentration derived from copper carbonate in the copper nanoparticles before reducing agent coating and the carbon concentration derived from the reducing agent. If the mass carbon concentration is less than 0.10%, sufficient dispersibility cannot be imparted to the reducing agent-coated copper nanoparticles, and a highly smooth coating film cannot be formed from a paste containing the reducing agent-coated copper nanoparticles. On the other hand, if the mass carbon concentration is greater than 2.00%, carbon residues are generated when the reducing agent-coated copper nanoparticles are fired, inhibiting low-temperature sinterability.

[0029] The mass carbon concentration of the reducing agent-coated copper nanoparticles can be measured using a carbon-sulfur analyzer (for example, "EMIA-920V" manufactured by Horiba, Ltd.).

[0030] The mass carbon concentration of the reducing agent-coated copper nanoparticles can be controlled by controlling the carbon concentration derived from copper carbonate in the copper nanoparticles before the reducing agent coating and the carbon concentration derived from the reducing agent. Of these, the carbon concentration derived from the reducing agent can be controlled by adjusting the reaction time in the reaction step and the number of washes in the washing step in the method for producing reducing agent-coated copper nanoparticles described below. The longer the reaction time in the reaction step, the higher the mass carbon concentration derived from the reducing agent. Also, the higher the number of washes in the washing step, the lower the mass carbon concentration derived from the reducing agent.

[0031] (Mass nitrogen concentration) The reducing agent-coated copper nanoparticles of this embodiment preferably have a mass nitrogen concentration of 0.50% or less. Here, "mass nitrogen concentration" refers to the mass ratio (mass%) of nitrogen to the total mass of the reducing agent-coated copper nanoparticles, and is the sum of the nitrogen concentration of the copper nanoparticles before reducing agent coating and the nitrogen concentration derived from the reducing agent in the case of a nitrogen-containing reducing agent (alkanolamine, alkylamine, amino-silane coupling agent). If the mass nitrogen concentration exceeds 0.50%, there is a greater concern that defects may occur in the fired film due to outgassing generated during firing. The lower limit of the mass nitrogen concentration is not particularly limited, and the mass nitrogen concentration may be 0.00% or more.

[0032] The mass nitrogen concentration of the reducing agent-coated copper nanoparticles can be measured using an oxygen / nitrogen analyzer (for example, LECO's "TC600").

[0033] The mass nitrogen concentration of the reducing agent-coated copper nanoparticles can be controlled by controlling the nitrogen concentration of the copper nanoparticles before the reducing agent coating and the nitrogen concentration derived from the reducing agent. Of these, the nitrogen concentration derived from the reducing agent can be controlled by the type of reducing agent (whether it contains nitrogen or not). In the case of a nitrogen-containing reducing agent, it can be controlled by adjusting the reaction time in the reaction step and the number of washes in the washing step in the method for producing reducing agent-coated copper nanoparticles described below. The longer the reaction time in the reaction step, the higher the mass nitrogen concentration derived from the reducing agent. Also, the higher the number of washes in the washing step, the lower the mass nitrogen concentration derived from the reducing agent.

[0034] (Mass oxygen concentration) The reducing agent-coated copper nanoparticles of this embodiment preferably have a mass oxygen concentration of 0.50% or more and 2.50% or less. Here, "mass oxygen concentration" refers to the mass ratio (mass%) of oxygen to the total mass of the reducing agent-coated copper nanoparticles, and is the sum of the oxygen concentration of the copper nanoparticles before the reducing agent coating and the oxygen concentration derived from the reducing agent in the case of an oxygen-containing reducing agent (alkanolamine, hydrosilane, amine-based silane coupling agent). If the mass oxygen concentration is less than 0.50%, the oxidation resistance of the copper nanoparticles is impaired, making it easier for oxidation of the copper nanoparticle surface to proceed in the air. If the mass oxygen concentration exceeds 2.50%, the effect of the reducing agent cannot sufficiently remove oxygen, and sintering at low temperatures is hindered.

[0035] The mass oxygen concentration of the reducing agent-coated copper nanoparticles can be measured using an oxygen / nitrogen analyzer (for example, "TC600" manufactured by LECO Corporation).

[0036] The mass oxygen concentration of the reducing agent-coated copper nanoparticles can be controlled by controlling the oxygen concentration of the copper nanoparticles before the reducing agent coating and the oxygen concentration derived from the reducing agent. Of these, the oxygen concentration derived from the reducing agent can be controlled by the type of reducing agent (whether it contains oxygen or not). In the case of a reducing agent containing oxygen, it can be controlled by adjusting the reaction time in the reaction step and the number of washes in the washing step in the method for producing reducing agent-coated copper nanoparticles described below. The longer the reaction time in the reaction step, the higher the mass oxygen concentration derived from the reducing agent. Also, the higher the number of washes in the washing step, the lower the mass oxygen concentration derived from the reducing agent.

[0037] The reducing agent-coated copper nanoparticles according to the present embodiment described above have excellent dispersibility in organic solvents even with a small organic content, so that a highly smooth coating film can be formed from a paste containing the copper nanoparticles. Furthermore, the reducing agent-coated copper nanoparticles according to the present embodiment have excellent sinterability at low temperatures.

[0038] [Method for producing reducing agent-coated copper nanoparticles] A suitable method for producing reducing agent-coated copper nanoparticles according to one embodiment of the present invention will be described. This method coats the surfaces of copper nanoparticles with a reducing agent through interaction between the surfaces of the copper nanoparticles and the reducing agent, and includes the following steps: component preparation, mixing / dispersion, reaction, washing, solid-liquid separation, and drying.

[0039] (Component Preparing Step) First, (i) copper nanoparticles having a coating at least part of the surface layer containing copper oxide and preferably further containing copper carbonate, (ii) a reducing agent, and (iii) an organic solvent are prepared.

[0040] (Mixing and Dispersing Step) Next, the copper nanoparticles, the reducing agent, and the organic solvent are mixed to obtain a mixture, and the mixture is subjected to a dispersion treatment to obtain a dispersion in which the copper nanoparticles are dispersed in the mixture. The dispersion method of the copper nanoparticles is not particularly limited, but one or more selected from a planetary mixer, a high-pressure wet disperser, an ultrasonic homogenizer, and an ultrasonic bath can be used.

[0041] The organic solvent is not particularly limited as long as it can dissolve the reducing agent and disperse the copper nanoparticles in. Examples of such organic solvents include one or more selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, terpineol, pentane, hexane, heptane, and octane.

[0042] (Reaction step) Next, in the reaction step, the resulting dispersion is stirred to coat the surface of the copper nanoparticles with the reducing agent. The stirring method for the dispersion is not particularly limited, but one or more selected from a planetary mixer, a high-pressure wet disperser, an ultrasonic homogenizer, and an ultrasonic bath can be used. The reaction step is performed while controlling the reaction temperature (dispersion temperature), and is therefore distinguished from the dispersion step in which temperature control is not performed.

[0043] The reaction time must be sufficient to coat the surface of the copper nanoparticles with the reducing agent. Furthermore, as mentioned above, the reaction time, together with the number of washes, is a factor that affects the mass carbon concentration (depending on the type of reducing agent, mass nitrogen concentration and / or mass oxygen concentration) of the reducing agent-coated copper nanoparticles. Therefore, it is necessary to appropriately determine the reaction time in combination with the number of washes so as to achieve the desired mass carbon concentration (depending on the type of reducing agent, mass nitrogen concentration and / or mass oxygen concentration). From these viewpoints, the reaction time is preferably selected appropriately from a range of 5 minutes to 180 minutes. The reaction temperature (dispersion temperature) is preferably 20°C or higher and 150°C or lower in order to sufficiently promote the reaction of coating the surface of the copper nanoparticles with the reducing agent.

[0044] (Washing step) Next, in the washing step, the dispersion liquid after the reaction step is subjected to solid-liquid separation, and an organic solvent is added to the obtained copper nanoparticle cake (solid side) to form a re-dispersion liquid, and the dispersion liquid is stirred at this time to wash the copper nanoparticles. The stirring method for the re-dispersion liquid is not particularly limited, but one or more selected from a planetary mixer, a high-pressure wet disperser, an ultrasonic homogenizer, and an ultrasonic bath can be used.

[0045] The organic solvent is not particularly limited as long as it can dissolve the reducing agent and disperse the copper nanoparticles in. Examples of such organic solvents include one or more selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, terpineol, pentane, hexane, heptane, and octane.

[0046] By carrying out the washing step, it is possible to remove the reducing agent that adheres without interacting with the surface of the copper nanoparticles. When the copper nanoparticles are made into a paste or ink, the reducing agent that adheres without interacting with the surface of the copper nanoparticles is liberated into the solvent and does not exhibit a reducing effect during firing. In addition, there is a concern that the reducing agent will generate outgassing during firing, causing defects such as voids and cracks.

[0047] As mentioned above, the number of washings is a factor that, together with the reaction time, affects the mass carbon concentration (depending on the type of reducing agent, mass nitrogen concentration and / or mass oxygen concentration) of the reducing agent-coated copper nanoparticles. Therefore, it is necessary to appropriately determine the number of washings in combination with the reaction time so as to achieve the desired mass carbon concentration (depending on the type of reducing agent, mass nitrogen concentration and / or mass oxygen concentration). From these viewpoints, it is preferable to appropriately select the number of washings from a range of 1 to 5 times. Note that the "number of washings" refers to the number of times this operation is performed, with one set consisting of solid-liquid separation of the dispersion, addition of an organic solvent, and stirring of the dispersion.

[0048] (Solid-liquid separation process) Next, the re-dispersed liquid after the washing process is subjected to solid-liquid separation to separate copper nanoparticles (solid side) and an organic solvent (liquid side). The solid side is a reducing agent-coated copper nanoparticle in which the reducing agent adhering without interacting with the surface of the copper nanoparticles is removed and the surface is coated with the reducing agent. The liquid side is an organic solvent in which the reducing agent that has not interacted with the surface of the copper nanoparticles is released, removed by the washing process. The solid-liquid separation method is not particularly limited as long as it can separate the copper nanoparticles and the organic solvent, and examples thereof include a supernatant removal method, a filtration method, and a centrifugation method.

[0049] (Drying step) Next, the cake of reducing agent-coated copper nanoparticles obtained by the solid-liquid separation step is dried to obtain a powder of reducing agent-coated copper nanoparticles. The drying conditions are not particularly limited, but from the viewpoint of sufficient drying, the drying temperature (ambient temperature) is preferably 20 ° C. or higher and 110 ° C. or lower, and the drying time is preferably 5 minutes or higher and 120 minutes or lower.

[0050] [Production of reducing agent-coated copper nanoparticles] 10.0 g of copper nanoparticles having an average particle size shown in Table 1 and at least a portion of the surface layer being a coating containing copper oxide and copper carbonate, 10.5 g of a reducing agent of the type shown in Table 1, and 4.5 g of 2-propanol as an organic solvent were weighed into a kneading vessel, and a dispersion was obtained by stirring and mixing using a planetary mixer.

[0051] The dispersion was transferred to a beaker and stirred using a magnetic stirrer. The reaction time was selected from the range of 10 to 120 minutes to control the mass carbon concentration (or mass nitrogen concentration and / or mass oxygen concentration, depending on the type of reducing agent) of the final reducing agent-coated copper nanoparticles. The reaction temperature was 25°C.

[0052] The dispersion obtained after the reaction was subjected to solid-liquid separation by filtration, and 15 g of 2-propanol was added to the resulting copper nanoparticle cake to form a re-dispersion. This re-dispersion was then stirred in an ultrasonic bath for 10 minutes for a washing step. The number of washings was selected from the range of 1 to 5 in order to control the mass carbon concentration (mass nitrogen concentration and / or mass oxygen concentration, depending on the type of reducing agent) of the final reducing agent-coated copper nanoparticles.

[0053] The washed re-dispersion was subjected to solid-liquid separation by filtration, and the resulting copper nanoparticle cake was dried in a dryer at 70 ° C. for 10 minutes to obtain a powder of reducing agent-coated copper nanoparticles. The mass carbon concentration, mass nitrogen concentration, and mass oxygen concentration measured by the above-described methods are shown in Table 1. In Table 1, hexyltrimethoxysilane used in No. 13 is listed in the "reducing agent" column, but does not fall under the category of "reducing agent" because it does not have a reducing action.

[0054] [Indirect Evaluation of Coating Smoothness] 0.1 g of the reducing agent-coated copper nanoparticle powder of each invention example and comparative example was added to 23.7 g of ethanol and dispersed using an ultrasonic homogenizer to obtain a dispersion. The particle size distribution of the reducing agent-coated copper nanoparticles in the dispersion was measured using a laser diffraction wet particle size distribution analyzer (Shimadzu Corporation "SALD-7100"). For each invention example and comparative example, the same sample was measured three times, and the average value D50 of the three median diameters calculated from the particle size distribution was calculated. This D50 was divided by the average particle diameter Dave of the copper nanoparticles (before reducing agent coating) listed in Table 1 to calculate the value "D50 / Dave" and is shown in Table 1. It has been found that the smaller the D50 / Dave, the better the dispersibility in organic solvents and the smoother the coating film can be formed from a paste containing copper nanoparticles. Therefore, a D50 / Dave of 2.5 or less was rated as "excellent," a D50 / Dave of more than 2.5 but less than 3.0 was rated as "fair," and a D50 / Dave of more than 3.0 was rated as "poor," and these were shown in the "Indirect evaluation of coating film smoothness" judgment column.

[0055] [Evaluation of Sinterability] 1.0 g of reducing agent-coated copper nanoparticle powder from each invention example and comparative example and 0.5 g of 2-propanol were weighed into a kneading container and kneaded using a planetary mixer to obtain a paste. The obtained paste was applied to a glass substrate and dried, and then fired at 250 °C for 1 hour in a nitrogen atmosphere using a reflow furnace (RSS-01 manufactured by Unitemp). The resistivity of the resulting fired film was measured using a tester (Digital Tester M-02N manufactured by CUSTOM) and is shown in Table 1. It is known that the lower the resistivity of the fired film, the better the sinterability of the fired film. Therefore, the resistivity of the fired film was measured using a tester (Digital Tester M-02N manufactured by CUSTOM) and the resistivity of the fired film was measured using a tester (Digital Tester M-02N manufactured by CUSTOM) and the results are shown in Table 1. It is known that the lower the resistivity of the fired film, the better the sinterability of the fired film. Therefore, the resistivity of the fired film was measured using a tester (Digital Tester M-02N manufactured by CUSTOM) and the results are shown in Table 1. －５ Ω cm or less is "Excellent", 5.0 x 10 －５ Ω cm or more 1.0 x 10 －４ Ω cm or less is "Acceptable", 1.0 x 10 －４ A value exceeding Ω·cm was rated as "poor" and shown in the "sinterability" column.

[0056]

[0057] The reducing agent-coated copper nanoparticles of the present invention can be used for joining electronic components, more specifically, for joining components such as substrates and elements in high-temperature environments where joining materials such as solder are difficult to use, such as in electronic devices called power devices.

Claims

1. Copper nanoparticles, the surface of which is a coating containing copper oxide and optionally copper carbonate, at least a portion of the surface layer of which is coated with a reducing agent, and the mass carbon concentration is 0.10% or more and 2.00% or less.

2. The reducing agent-coated copper nanoparticles according to claim 1, having a mass nitrogen concentration of 0.50% or less.

3. The reducing agent-coated copper nanoparticles according to claim 1 or 2, having a mass oxygen concentration of 0.50% or more and 2.50% or less.

4. The reducing agent-coated copper nanoparticles according to claim 1 or 2, wherein the reducing agent comprises one or more selected from the group consisting of alkanolamines, alkylamines, hydrosilanes, and amine-based silane coupling agents.

5. The reducing agent-coated copper nanoparticles according to claim 1 or 2, wherein the average particle size of the copper nanoparticles is 50 nm or more and 500 nm or less.

6. The reducing agent-coated copper nanoparticles according to claim 3, wherein the reducing agent comprises at least one selected from the group consisting of alkanolamines, alkylamines, hydrosilanes, and amine-based silane coupling agents.

7. The reducing agent-coated copper nanoparticles according to claim 3, wherein the average particle size of the copper nanoparticles is 50 nm or more and 500 nm or less.

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